The use of electronic color generation circuitry for converting digital image data to color video signals in order to display color images on a cathode ray tube or the like is an art recognized concept. Known prior art systems typically employ a color signal generator consisting of three memory devices corresponding to the primary colors of red, blue and green, each of which memory devices have color reference data stored therein. The color reference data may be read from memory and combined to produce an additive color video display which is characterized by a predetermined, somewhat arbitrary set of values of intensity, hue and saturation. Intensity (brightness) relates to the luminance of the color, and saturation characterizes the purity of the color, i.e., the extent to which it is mixed with white, while hue relates to the dominant wave length of color. The color reference data output from the three memory devices is converted to analog signals which are employed as the red, blue and green video signals to form an additive color image on a cathode ray tube. Thus, a given set of digital image data delivered to the three memory devices, which are commonly referred to in the art as "look-up tables," results in a color image whose hue and saturation are determined by the relative proportions of red, blue and green video signals derived from the respectively associated look-up tables while the perceived intensity of such color image is determined by the sum of these three primary colors. Prior art devices have included means for allowing a user to alter the digital image input data for the purpose of changing the colors in the resultant color video image (which also incidentally changes the values of intensity, hue and saturation of such color video image), however, for reasons discussed below, the resulting changes in the color video image produced undesirable results as perceived by a viewer.
The undesirable results mentioned above are related to the fact that the color characteristics of intensity, hue and saturation are not simple functions of the red, blue and green color levels, but are highly interdependent and are interrelated by complex mathematical formulae. For example, hue and saturation are complex ratio functions of the primary color levels, while intensity is a function of the sum of such color levels. These relationships are further complicated by the nonlinear response or "gamma" of television systems. Although in the past a user has had the flexibility to alter or transform the source image data in a manner to change the levels of red, blue or green color levels, it was extremely difficult, if not completely impossible, to predict the particular combinations of intensity, hue and saturation which would result from such alteration of the source image data. Thus, for example, it was heretofore impossible for a user to change the resulting color video display from one hue to another which was at the same perceived saturation and intensity. Similarly, it was not possible to change the intensity of the display without also changing the hue or saturation thereof, or to change the saturation level of the display without also changing hue and intensity. This inability to independently alter the perceived color characteristics of intensity, hue and saturation was a significant disadvantage, since the capability to independently control intensity, hue and saturation of a color image provides additional flexibility in performing significant analytical and diagnostic operations with color television systems.